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FISH HABITATS IN LOCH NESS


Reproduced with the permission of the Scottish Naturalist
Copyright: May be used for private research. All other rights reserved

 

 

By ADRIAN J. SHINE

Loch Ness and Morar Project

 

JAN KUBECKA

Hydroacoustic Unit, Department of Biology,

Royal Holloway University of London

 

DAVID S. MARTIN

Loch Ness and Morar Project

 

ANNIE DUNCAN

Hydroacoustic Unit, Department of Biology,

Royal Holloway University of London

 

Introduction

Obtaining reliable ecological data on the fish species living in a large water body the size of Loch Ness is no easy task, and depends upon employing a variety of sampling methods.  Apart from sampling difficulties associated with the size and depth of this water body, live capture by means of seine-nets, trawl-nets and tow-nets exerts a simpler mechanical selectivity within the area of collection than do passive techniques using gill-nets or fyke-nets, with which more complex behavioural selectivity plays a bigger role.

The above conventional fish capture techniques were used in Loch Ness from 1988 onwards at sites indicated in Figure 1 (5K), and each method was more appropriate for some fish habitats available in Loch Ness than for others (Figure 2a, 28K  and Figure 2b, 18K), thus introducing another level of selectivity.  To capture fish in the profundal zone, unconventional methods had to be adopted, such as setting gill-nets at 200 m depth with their lead-line resting on the bottom sediment.  The aim of this paper is to add the more recent results, from a greater variety of capture methods, to the fish habitat picture published by Shine and Martin (1988).

Vol 105, The Scottish Naturalist: Fish Habitats in Loch Ness. p239 

Methods

Pelagial Habitat

The Loch Ness and Morar Project (L.N.M.P.) gill-net series used to sample the pelagial habitat consisted of five nets between 20 and 25 m long, 1.25 m deep, and with mesh bar sizes of 20 mm, 25 mm and 30 mm, giving a total length of 115 m and a fishing area of 1432.  These were suspended on the L.N.M.P. mooring rope in the North Basin opposite Brachla
(see Figure 1), in mid-loch between 20 and 30 m depth, in order to span the scattering layer (Shine and Martin, 1988).  During the July-August 1991 series, the nets were exposed on fifteen dates, during daylight hours, at night or over 24 hours.

 

In October 1991 and May 1992, six gill-nets (30 m x 6.0 m; with 15.5 mm, 25 mm and 30 mm mesh sizes) belonging to Royal Holloway University of London (R.H.U.L.) were suspended on the same L.N.M.P. mooring rope, at depths between 20 and 30 m, to encompass the scattering layer which had been detected acoustically.  This net series was 180 m long, and had a fishing area of 1,080 m2.  In October 1991, gale force winds prevented the collection of the nets after 24 hours and they were left for four days.  The same nets were left on two occasions for 24 hours in May 1992, when the weather was calmer.  Also during May 1992, an ichthyoplankton tow-net of 1.0 m diameter and with a 1.0 x 1.7 mm mesh size was towed at three knots within the scattering layer at about 24 m depth for two hours.

 

On two occasions, in August 1988 and November 1992, the Department of Agriculture and Fisheries for Scotland's (D.A.F.S.) vessel operated a trawl-net in the South Basin at mid-water depths down to 30 m.  The August sample was collected using a sprat-trawl with a 12 mm mesh cod-end and a 6.0 m mouth opening (four hauls totalling four hours of trawling).  The November sample was collected in the fine-meshed cod-end during trawling trials between Fort Augustus and Urquhart Bay.

 

Sublittoral Habitat

During December 1992, the L.N.M.P. gill-nets together with some Greer gill-nets (bar sizes 19-30 mm and area 121 m2, giving a total area of 264 m2) were set to sample sublittoral depths of 15-25 m off Brachla.  The nets were set parallel to the shore with their lead-line resting on the bottom sediment.  The same technique

Vol 105, The Scottish Naturalist: Fish Habitats in Loch Ness. p242

 

Vol 105, The Scottish Naturalist: Fish Habitats in Loch Ness p243

was used to collect the fish as in the Table headed 'Littoral Netting 1984' in Shine and Martin (1988: 148).  On a few occasions, a gill-net (0.5 m high and 200 m long, with a bar size of 15 mm) was deployed down the loch walls from depths of 30 m to 200 m in order to sample any fish present.

Profundal Habitat

The same series of gill-nets were used to sample the fish inhabiting the profundal at 200 m depth, both in Shine and Martin (1988) and in subsequent sampling .  The nets were set with their lead-line resting on the bottom sediment.  Occasionally, additional gill-nets or fyke-nets were added (Figure 2b).  More nets were deployed during the five dates in 1991 (396 m2) than during the seven dates in 1988 (35-135 m2), which gave a better success rate and fewer occasions with zero catches.

For comparative purposes a gill-net series, consisting of both the Greer and the 0.5 m high and 200 m long nets, combined with a series of fyke-nets, were set in Loch Morar in August 1992 for two days at a depth of 275 m, but with no fish caught.

Littoral Habitat

During May-June 1992, another technique was employed to sample littoral fish during the day and night.  An R.H.U.L. beach seine-net, 15 m long and 2.5 m deep with 1.0 cm mesh size, was laid parallel to the shore-line and about 15-20 m out into the water.  After setting the net, the two ends of the net were pulled gently, symmetrically and quietly by rope.  Particular attention was paid to the lead-line in order that it be pulled close to the bottom.  The area sampled per haul was calculated according to Kubecka and Bohm (1991) to be 80 m2 for beaches near Brachla (29th/30th May 1992) and 117 m2 at Dores Beach (6th/7th June 1992).  In all, fourteen hauls were taken and all 272 fish caught were released within fifteen minutes of capture, after measurement and removal of scales for ageing.

Results and Discussion

The same seven species of fish were recorded for Loch Ness during this period of sampling as were recorded by Shine and Martin (1988).  This is the same number as given by Maitland and his colleagues (Maitland, 1981), who included Pike Esox lucius, but not Minnows Phoxinus phoxinus which were first recorded

Vol 105, The Scottish Naturalist: Fish Habitats in Loch Ness p244

for Loch Ness in 1985-87 (Shine and Martin, 1988).  Pike were not captured during the period 1988-92, although some were caught in the sublittoral in previous years.  Maitland (1981) also identified his lamprey as the Brook Lamprey Lampetra planeri.

Pelagial Habitat

The numbers and sizes of fish species caught by the various collecting methods are given in Table 1.  During fifteen deployments between July and September 1991, the L.N.M.P. gill-nets suspended at about 20-30 m depth caught 33 Charr Salvelinus alpinus and a few Brown Trout Salmo trutta.  There were two size classes of Charr, with modal lengths of 16 cm and 26-27 cm.  All were adults, with 24 females and ten males.  The average catch per unit effort was 2.0 fish per 24-hour exposure.

In October, when the R.H.U.L. gill-netting (1,080 m2) was left fishing between 20 and 30 m depth for four days, there was a large catch of 17 Charr, ten of which were juveniles (<12 cm fork length), and 23 adult Brown Trout up to 2.0 kg in weight, ten of which were Ferox Trout.  It seemed likely that the Trout were attracted to the captive Charr, since some Trout were trapped within inches of the Charr, some of which had been chewed.  Fewer fish, and only Charr of a similar size range, were caught during a 24-hour exposure of the same nets at the same site in May 1992.  Most of these Charr were adults, and some were mature - one female with large eggs in the ovary and one male with large testes.

Both fish catches by the D.A.F.S. vessel using a mid-water trawl in the South Basin are intriguing on two counts.  Both the August 1988 and the November 1992 trawling collected both Three-spined Sticklebacks Gasterosteus aculeatus and very small Charr from the full pelagial in waters down to 30 m depth.  The collection of as many as 50 Sticklebacks in November 1992 is a surprising record, which should be investigated further because of its implications for acoustic studies (Kubecka, Duncan and Butterworth, 1993).  Both D.A.F.S. trawl hauls captured large numbers of Charr, some of which were the smallest Charr (lengths of 3.0, 4.5 and 5.0 cm) so far recorded from Loch Ness, or indeed, as far as we are aware, from the pelagic of any Scottish loch.

Mr. R.B. Greer of D.A.F.S. identified these Charr as young but not larval fish, as might be expected if Loch Ness Charr spawn in the loch in late October/early November.  Moreover, larval Charr were not collected in the plankton during

Vol 105, The Scottish Naturalist: Fish Habitats in Loch Ness p245

autumn 1992 by tow-netting within the scattering layer, nor during May 1992, as might be expected if the Charr were spring spawners.  Further study is urgently needed on the spawning sites and the life cycle of the Charr, which is probably the numerically dominant species in Loch Ness.  A tow-net larger than 1.0 m diameter may be an advantage.  The question as to whether Charr spawn on beaches in the loch or in local rivers also needs to be resolved.  There are no known records, however, of Charr entering any feeder stream of Loch Ness.

The capture of the larval Lamprey in May 1992 is a new pelagial record for Loch Ness, although there is also a recent profundal record (Martin, Shine and Duncan, 1993).  It was possibly an individual drawn into the pelagial by wind-induced currents.

Sublittoral Habitat

The collection of mature Charr, whose gonads were 'running' ripe, during December 1992 by the L.N.M.P. gill-net series, set at sublittoral depths of 15-25 m off Brachla, adds some supportive evidence for autumn spawning in the loch, and may indicate the location of one possible spawning site.  This was an unusual catch, since regular sampling of the sub-littoral depths between 10 and 30 m with the same nets during May-June 1989 and March-September 1991 normally caught more Brown Trout than Charr.

Profundal Habitat

Sixteen more Charr were caught from the profundal regions on twelve sampling occasions during the 1988-92 period, thus giving an overall total of 29 profundal fish caught since 1982, with standard lengths ranging from 4.5 to 27.9 cm.  These were fish with profundal invertebrates in their guts.  About a third of these fish were less than 10 cm in length, and about 40% were greater than 19 cm.  The fish were caught either by benthic trawling or with an adequate area of gill-netting (about 400 m2) in deep water.

Littoral Habitat

Night-seining is one of the most cost-effective sampling techniques, which exploits the behavioural tendency of many fish species to migrate shorewards during twilight (Kubecka, 1993).  The technique thus samples both the littoral fish, which spend their days in shallow water, as well as the deep or open-water fish,

Vol 105, The Scottish Naturalist: Fish Habitats in Loch Ness p248

 

Vol 105, The Scottish Naturalist: Fish Habitats in Loch Ness p249

which move shorewards at dusk.  It proved valuable in Loch Ness, where it was used for the first time in May-June 1992.

Table 2 lists the species composition of fish caught.  During the day, no fish were caught in four out of five hauls, probably because the well-lit shallow water is a dangerous place for small fish.  Nevertheless, small fish, such as Minnows, Sticklebacks and even small salmonids, form quite dense shoals in very shallow sites, thus avoiding predation by big fish (see the selective day catch in Table 2).

At night, Sticklebacks were caught in large numbers in every haul, together with small salmonids and Minnows in some hauls.  The numbers of salmonids caught increased with 'true darkness', but declined abruply with the advent of 'dawn', which arrives early in the short northern night (Figure 3, 4K).  It would be advantageous to repeat this exercise when nights are longer.

Although only 29 Brown Trout were caught, Figure 4 (6K) shows that three year classes of fish were present, namely 1+, 2+ and 3+.  With a bigger net, it seems likely that night shore-seining could be used to establish the ratios of young Brown Trout to young Salmon Salmo salar to littoral Charr, which is useful information for understanding the ecology of Loch Ness salmonid populations.

Before embarking on a longer term study, some care should be taken with the size of net used.  The number of fish caught is a function of the area sampled quantitatively, and Figure 5 (5K) illustrates the relationship between net length and area sampled.

Sticklebacks and Minnows are an important food source for piscivorous salmonids, so any information about their population size structure is useful.  Figure 6 (6K)  shows that three size groups were present in the Stickleback population sampled by shore-seining; these were 31-33 mm, 41-45 mm and 51 mm or larger.  Although Sticklebacks cannot be aged by their scales, further study on their growth rates would provide this information.

No Pike were caught by shore-seining in Loch Ness, perhaps because Pike were absent in the sampled areas, whereas the catch per unit effort of small Eels Anguilla anguilla (30-35 cm) was high compared with other localities where night-seining has been carried out (e.g. River Thames, Czech and London reservoirs).  Both species need further study in Loch Ness.

Vol 105, The Scottish Naturalist: Fish Habitats in Loch Ness p253

 

Summary of Fish Habitats in Loch Ness

Sampling Loch Ness fish habitats is a difficult task.  Table 1 gives a summary of the species composition and sizes of the fish catches in the different habitats of the loch.  Charr is the most ubiquitous species in the loch and the most abundant species in the pelagial and the profundal.  Brown Trout dominate sublittoral catches with Charr coming second (see also Shine and Martin, 1988), whilst the Three-spined Stickleback is an important species in the littoral and the pelagial (Figure 7, 46K colour chart).

The various fish habitats were sampled by appropriate methods, which are illustrated in Figure 2a and Figure2b.  At present, depths greater than 50 m in the open water have not been sampled by live capture techniques, and depths between 50-70 m and 200 m were not reached by 420 kHz dual-beam sonar (Kubecka, Duncan and Butterworth, 1993).  Since then, however, a 38 kHz split-beam sonar has shown a small number of probable fish targets at these depths, and a greater number on the loch bed (Figure 8, (19K colour chart).  Having established a sampling technology for some of these fish habitats, and with the aid of quantitative echo-sounding (Kubecka, Duncan and Butterworth, 1993; Shine, Martin and Marjoram, 1993), the situation is ripe for studying how the more numerous species, Charr and Sticklebacks, exploit this deep oligotrophic loch, by finding out where they spawn, where their subsequent life-cycle stages live, and quantifying their impact upon the ecosystem.

Acknowledgements

We should like to express our thanks to the Royal Holloway University of London for permission to use the fish sampling equipment, and to the Loch Ness Fisheries Board for permission to try out whether night shore-seining was a useful fish sampling method in one of Britain's largest lochs.  We also acknowledge the presence and help of the bailiff, Mr. W. Hastings.

 

References

Kubecka, J.  (1993).  Night inshore migration and capture of adult fish by shore seining.  Aquaculture and Fisheries Management, 24: in press.

Kubecka, J. and Bohm, M.  (1991).  The fish fauna of the Jordan Reservoir, one of the oldest man-made lakes in central Europe.  Journal of Fish Biology, 38: 935-950.

Vol 105, The Scottish Naturalist: Fish Habitats in Loch Ness p255

Kubecka, J., Duncan, A. and Butterworth, A.J.  (1993).  Large and small organisms detected in the open waters of Loch Ness by dual-beam acoustics.  Scottish Naturalist, 105: 175-193.

Maitland, P.S.  (Ed.)  (1981).  The Ecology of Scotland's Largest Lochs: Lomond, Awe, Ness, Morar and Shiel.  Monographiae Biologicae, Vol. 44.  The Hague: Junk.

Martin, D.S., Shine, A.J. and Duncan, A.  (1993).  The profundal fauna of Loch Ness and Loch Morar.  Scottish Naturalist, 105: 113-136.

Shine, A.J. and Martin, D.S.  (1988).  Loch Ness habitats observed by sonar and underwater television.  Scottish Naturalist, 100: 111-199.

Shine, A.J., Martin, D.S. and Marjoram, R.S.  (1993).  Spatial distribution and diurnal migration of the pelagic fish and zooplankton in Loch Ness.  Scottish Naturalist, 105: 195-235.

  

Received July 1993

Mr. Adrian J. Shine, Loch Ness and Morar Project,

Loch Ness Centre, DRUMNADROCHIT, Inverness-shire IV3 6TU.


Dr. Jan Kubecka, Hydrobiological Institute,

Czech Academy of Sciences, 370 05 Ceske Budejovice, Na Sadkach 7, CZECH Republic.

Present address: Hydroacoustic Unit, Department of Biology, Royal Holloway
University of London, EGHAM, Surrey TW20 0EX.


Mr. David S. Martin, Loch Ness and Morar Project,

Loch Ness Centre, DRUMNADROCHIT, Inverness-shire IV3 6TU.


Dr. Annie Duncan, Hydroacoustic Unit, Department of Biology,

Royal Holloway University of London, EGHAM, Surrey TW20 0EX.

 

 

 

 

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